The present invention relates to a multipath propagation delay determining means, in particular for a CDMA base station, in which pilot symbols contained periodically in the signal radio frames are used for an efficient power delay profile calculation and an improved path selection, tracking and sector selection.
In particular, the invention relates to performing the afore-mentioned functions in connection with a so-called RAKE receiver.
Code-division multiple access (CDMA) based on direct-sequence (DS) spread-spectrum (SS) techniques is a prospective candidate for the third generation of wideband cellular mobile telecommunication systems (e.g. in UMTS, as IMT-2000 described in reference [1]: J. E. Padgett et al: xe2x80x9cOverview of Wireless Personal Communicationsxe2x80x9d, IEEE Communications Magazine, January 1995, pages 28-41).
As shown in FIG. 1 an area, where several mobiles stations MS1, MS2 . . . MS are served by a (fixed) base station BS, can be regarded as a cell of the CDMA communication system. It has already been demonstrated that the DS-SS CDMA technique is capable of transmitting data signals of high transmission speed, for example within RACE CODIT project (reference [2]: A. Baier et al: xe2x80x9cDesign Study for the CDMA-based Third Generation Mobile Radio Systemxe2x80x9d, IEEE Journal on Selected Areas and Communications Vol. 12, May 1994, pages 733-743). The potential advantages of the DS-SS CDMA technique have also been tested in the Ericsson Wideband-Testbed (WBTB) project. DS-SS-CDMA has already been used in commercial systems like systems based on IS""95 (D. P. Whipple: xe2x80x9cThe CDMA Standardxe2x80x9d, Applied Microwave and Wireless, December 1994, pages 24-37). Also in Japan a great importance has been attributed to the DS-SS-CDMA system.
Whilst some basic properties of the CDMA receiver and the CDMA telecommunication system are implicit due to the CDMA method, special realizations of the despreaders, the searchers and path selection units have not been investigated in a great detail up to now, since a standard for the W-CDMA has so far not been established. Therefore, the present invention relates to special realizations of the individual units necessary in the CDMA-receiver. Since the inventive CDMA base station, the CDMA reception method and the CDMA system are intrinsically based on the DS-SS CDMA technique, hereinafter the basic technique of DS-SS CDMA transmission will be considered (see also the basic reference [4]: A. J. Viterbi: xe2x80x9cCDMA: Principles of Spread Spectrum Communication, Reading, Mass.: Adison-Wesley, 1995xe2x80x9d).
DE 19506117 C1 describes a method for estimating the impulse response of a transmission channel, over which CDMA-method-coded information is transmitted. The information is spread on the transmitter side with a spreading code and is despreaded on the receiver side with a corresponding corelation code. The temporal changes of the propagation paths are taken into account on the receiving side.
DE 19615257 A1 describes a CDMA-RAKE-receiver including a sub-chip-resolution. This receiver is adapted for use in a DS-CDMA-communication system. It includes a channel estimation means which can resolve multipath-components, which are closer than a single chip-interval.
Basic CDMA-Technique
Basically, in the CDMA technique, an input signal I having a limited bandwidth (transmission speed) is spread with a predetermined spreading sequence (PN sequence) of a much higher bandwidth and thus an output signal O is produced having a much higher bandwidth than the input signal I as is shown in FIG. 2a. Since all signals considered in the CDMA technique are digital signals, the expression xe2x80x9cbandwidthxe2x80x9d really means the chip rate.
As is shown in FIG. 2b, two bits of a digital signal constitute one symbol in a CDMA method using a QPSK modulation. Each bit of the symbol will be spread with a PN sequence, and the spread signal (the bottom curve in FIG. 2b) consists of a plurality of xe2x80x9cchipsxe2x80x9d, whereby a chip is defined as a 0xe2x86x921 and 1xe2x86x920 (or 1xe2x86x920 and xe2x86x921) portion of the despread signal.
As indicated in FIG. 2a, a so-called spreading gain M equal to the ratio of the chip rate to the symbol rate is defined. M basically describes the spreading factor, i.e. how much wider the bandwidth has become due to the spreading with the PN sequence. Of course, since all signals are digital also the PN sequence is a signal which is digital (consisting of a number of bits).
If the original signal I has to be recovered in the CDMA receiver, of course a despreading process has to be carried out in a despreader DSP as shown in FIG. 2a, wherein the original information is obtained by multiplying the spread signal (sequence O) with the original PN sequence that was used for the spreading process.
However, as shown in FIG. 3, all information in the CDMA channels are transmitted clockwise, i.e. in terms of successive radio frames RFn. This means, that the spreading and despreading must be performed also framewise. In the transmitter, each frame is spread with the spreading sequence (PN sequence) starting with the beginning of the frame and of course this means that also in the receiver there must be a time synchronized (i.e. time-aligned) despreading, i.e. the despreading sequence must be aligned to the beginning of the received frame. The PN sequence is of course a sequence which is known to the transmitter and receiver, but the time-alignment for the block-wise (M) integration (despreading) must be performed in the receiver.
A principle overview of a base station receiver is shown in FIG. 4. As is seen in FIG. 4, the demodulator DEMOD receives inputs from the PN generator PN-GEN (generating the PN despreading sequence) and from a timing control unit TCU.
In principle, signals from various antennas Ant0, Ant1 from various sectors 1 . . . 6 are input to an automatic gain control circuit AGC and the samples are input to a so-called searcher S (the function of which will be explained below) which calculates the (power) delay profiles. The demodulator DEMOD (comprising a so-called RAKE receiver to be explained below in more detail) outputs the demodulated and despread bit sequence to the decoder DEC. As will be seen below, the searcher S actually comprises a searching and tracking unit provided for input signals from all sectors (parts of a cell as shown in FIGS. 1, 12). The output from the searcher S are the delay values and the (sector) selection information.
The reason why the searcher S also comprises a tracking unit results from the problem of multipath propagation which is an intrinsic property of any mobile communication system. Therefore, hereinafter the multipath propagation in connection with the tracking features of the CDMA system are explained.
CDMA Multipath Propagation
As shown in FIG. 5, between a mobile station MS and a base station BS there is not only the direct path P1 but also indirect paths P2, P3, for example due to reflections at buildings H, cars C or mountains M. This mixture of direct and indirect paths (i.e. multipath propagation) means that the received signal energy (i.e. the power per sample of the transmitted sequence) does not have a constant time delay (corresponding to the velocity of light). This means, that a sample (bit) transmitted at t0 arrives at the base station BS at the time t1 and another portion of the energy arrives at the base station BS at time t2 due to a further propagation of the energy along an indirect path P2 or P3. This leads to the delay profile per sample as is illustrated in FIG. 5. That is, each sample is spread over the delay profile, often characterized by (fading) single paths. Thus, in FIG. 5 the time differences t1xe2x88x92t0, t2xe2x88x92t0 etc. are defined as delays d1, d2 etc.
In conventional DS-SS-CDMA techniques the problem of multipath propagation is usually handled by the so-called RAKE receiver as is described in the afore mentioned references [2] and [3]. The basis of the RAKE receiver is basically to collect the energy per symbol not only from the direct path P1 but also from the plurality of indirect paths P2, P3. Essentially the RAKE receiver allocates a xe2x80x9cmarkerxe2x80x9d (in CDMA such markers are called xe2x80x9cfingersxe2x80x9d) to the strongest single paths (i.e. to the maxima) in the delay profile of the corresponding signal. Thereafter, the collected energy or the information of each path is individually demodulated/detected per path (i.e. per RAKE finger). Thereafter the information after demodulation is combined, e.g., with a maximum-ratio-technique.
If the mobile station MS with respect to the base station BS was stationary, then of course the delay profile with respect to also stationary reflection objects H, M could be pre-estimated and calculated. However, one of the intrinsic properties of a mobile radio communication network is the xe2x80x9cdynamicxe2x80x9d variation of the delay profile when the mobile station MS or one of the non-stationary objects C move. Therefore, also the delay profile exhibits a dynamic characteristic. Thus, the resource allocation and the time synchronization of the RAKE receiver has to be performed by continuously estimating and evaluating the delay profile.
In the CDMA technique a so-called searching and tracking unit is normally used to identify the paths within a delay profile.
Searching and Tracking Unit
A major task of the searching and tracking unit is to identify the paths within a delay profile and keep track of changing propagation conditions, e.g. as a consequence of distance variations between the mobile station MS and the base station BS. Since in the base station receiver the despreading sequence must be fully time-aligned to the sample (energy) arriving at the base station BS along a plurality of paths, it is essential that the searching and tracking unit knows the relative delays d1, d2, . . . dp of the paths within the delay profile. If so, the requested time-synchronization for each RAKE finger can be maintained. Therefore, the searching and tracking unit must on the one hand estimate the delay profile and on the other hand must assign the RAKE fingers accordingly in order to time-align the PN despreading sequence to the exact arrival time of the partial sample energy arriving over each individual path.
Often a certain frame structure with fixed alignment of information signal (frames) and spreading sequences is applied and therefore the time-synchronization can be split up into a frame-synchronization and chip-synchronization. As a consequence of fading and changing propagation conditions the estimation of the delay profile carried out by the searching unit has to be updated according to the specific needs of the mobile radio channel.
Therefore, the searcher has to fulfill two contradictory requirements, namely, on the one hand it must minimize the time needed for updating or calculating the exact delay profile and on the other hand it must provide a sufficiently fine time resolution for time-aligning the PN despreading sequence to the beginning of the respective frame or symbol, i.e. to minimize the self-noise of the PN sequence.
Conventional Searching and Tracking Unit
Prior art searcher algorithms and implementations in communication applications mainly relate to IS-95 (commercial) systems, either for the up-link (MSxe2x86x92BS) as described in reference [4] and reference [5]: K. Easton and J. Levin: xe2x80x9cMultipath Search Processor for a spread Spectrum Multiple Axis Communication Systemxe2x80x9d, WO 96/10873, Apr. 11, 1996xe2x80x9d or for the down-Link (BSxe2x86x92MS) in reference [6]: R. Blakeney et al xe2x80x9cDemodulation Element Assignment in a System Capable of Receiving Multiple Signals, WO 95/12262, May 4, 1996xe2x80x9d.
As already shown in FIG. 3, each superframe SRF consists of a number of radio frames RFn which each consist of a number of time slots TSm. Each time slot TSm has a number of pilot symbols PS2 which allow to detect the beginning of the time slot TSm. Therefore, the pilot symbols can be used in order to achieve the time-aligning of the PN despreading sequence to the beginning of the individual time slots.
In order to achieve a high system capacity, the prior art according to the IS-95 systems do not use pilot symbols in the up-link channel. If the pilot symbols are not contained, the searchers must examine all possible signal variations which random data can produce and perform the delay profile calculation on the basis of such an estimation. In the downlink channel for example in the Ericsson WBTB system a continuous pilot signal is inserted. The up-link delay estimation is based on a decision feedback.
Prior Art Searching and Tracking Unit
As described in WO 96/10873 a typical receiver uses multiple searcher elements working in parallel to provide a fast searching process. Such a searching and tracking unit comprising a plurality of searchers S is shown in FIG. 6. As shown in FIG. 6, a plurality of searchers S1 . . . SL work in parallel as a consquence of the multiple signal sources (antennas from each of the sectors 1 . . . 6) which should be examined. The parallel operation is also a consequence of the xe2x80x9creal timexe2x80x9d requirements. Namely, if a real-time serial search is applied, for each new time offset, (code phase imcrement since in the CDMA method each channel is identified by a respective time offset to a synchronization pulse) an additional correlation (dwell) time must be spent.
To avoid this xe2x80x9creal-time slaveryxe2x80x9d, WO 96/10873 suggests a new hardware architecture for the searcher. The essence of the new searcher architecture is to de-couple the operation of the correlator (based on a Fast Hadamard Transform-FHT processor) from the real time requirements by introducing a buffer for the input signal samples and a PN sequence buffer for the desprading sequences. In this way the FHT processor can run at much higher speed evaluating quickly the large number of time offsets with respect to the reference (synchronization) signal. In WO 96/10873 an efficient technique for supplying high speed data streams to the FHT processor is included. The hardware architechture is similar to the one implemented within CODIT and the WBTB test project of Ericsson. The WBTB approach can further be described as a combination of coherent accumulations with non-coherent averages in order to reduce the variance of estimates.
Path Selection Unit
As is also shown in FIG. 6 for the conventional system (see for example WO-96/10873) in addition to the parallel working searchers S1 . . . SL there is a path selection unit PSU that selects the individual paths from the calculated power delay profiles as determined by the set of searchers. As is seen in FIG. 5, the delay profile has a number of peaks and the path selection is conventionally done by scanning the calculated delay profile for a certain number of strongest peaks, whereafter these peaks are compared to a threshold, which is derived by multiplying the xe2x80x9cnoise floorxe2x80x9d of the delay profile with a constant value.
The disadvantage with such a kind of path selection is that it is not very accurate, in particular when cells are used, which are subdivided into sectors, and when multiple antennas per sector (antenna diversity) is used.
As described with reference to FIG. 3 above, each time slot comprises a number of pilot symbols and over the successive time slots it may be said, that the pilot symbols are periodically inserted (after each 0.625 ms). Each logical channel (information) corresponds to one voice or packet data channel. In a commercially interesting system up to 300 voice channels per base station must be handled simultaneously. This means, that each voice or packet data channel has to simultaneously undergo the delay profile estimation and the updating of the delay profile simultaneously for which the PN despreading sequence must be appropriately time-aligned to the beginning of the respective time slot.
The above described solution of estimating the absolute delays is not optimal for the CDMA systems with periodically inserting pilot symbols. On the other hand, another solution suggested in the Ericsson WBTB project proposes that a long buffer is used that is able to reflect all possible delay values within a cell. The hardware in such a system is extremely complex when 300 voice channels per base station need to be handled, since essentially 300 parallel working searchers must be provided.
Therefore, a first aim of the invention is to provide a multipath propagation delay determining means, in particular for the DS-SS-CDMA base station receiver where no complex hardware is necessary for the searchers, whilst still an accurate estimation of the power delay profile for a high number of voice channels can be achieved in real time.
As also described above, one of the most important general problems is to select the individual paths from the delay profile, since the estimation of the delay values is necessary in order to solve the problem of the multipath propagation. In the conventional path selection unit a threshold is set for the discrimination between the signal and the noise. Furthermore, reference [7]: E. S. Sousa, V. M. Jovonvich and C. Daigneault, xe2x80x9cDelay Spread Measurements for the Digital Cellular Channel in Torontoxe2x80x9d, IEEE Transactions on Vehicular Technology, Vol. 43,. No. 4, pages 837-847, November 1994xe2x80x9d contains a description of a modified threshold setting method for the channel delay profile estimation using a so-called constant false alarm rate technique (CFAR). However, this method is extremely complex and is more suitable for off-line signal processing and does not fulfill the requirements of the real-time implementation in a commercially interesting CDMA telecommunication system.
Therefore, another aim of the invention is to provide a multipath propagation delay determining means, in particular for a DS-SS-CDMA base station receiver, in which an accurate path selection estimation in a real-time application can be performed.
Furthermore, as described above, in the CDMA system cells may be subdivided into sectors and multiple antennas may be used per sector (antenna diversity). Thus, the accuracy of the delay profile estimation and the treatment of the softer (i.e. sector-wise) handover has to be specified and optimized with respect to the special requirements of a hardware with as low complexity as possible.
Therefore, a further object of the invention is to provide a multipath propagation delay determining means, in particular for a DS-SS-base station receiver, allowing an accurate delay profile estimation and a softer handover when cells are subdivided into sectors in which an antenna diversity is used.
The above aims can be summarized in a single object of the invention, i.e. to provide multipath propagation delay determining means, in particular for a DS-SS-CDMA-base station receiver, in which a great number of voice or packet data channels with periodically inserted pilots symbols can undergo in real-time simultaneously an accurate despreading, an accurate delay profile estimation as well as an accurate path selection and location determination.
Solution of the Object
The above object is solved by a multipath propagation delay determining means, for a CDMA base station receiver.
Essentially, as a primary aspect of the invention an improved power delay profile is calculated according to the invention by averaging delay profiles estimated over a plurality of successive time slots and frames. Further aspects of the invention are claimed in the dependent claims.
One preferred aspect of the invention is how the delay profile is searched for local maxima corresponding to the individual paths. Here, the peaks in the profile are removed or equivalently set to 0 to obtain a noise floor. This noise floor is averaged to result into a single value. Then a threshold factor is multiplied with this noise floor level. Then the original unmodulated delay profile is compared with the multiplied value and those maxima are selected as useful paths which lie above the multiplied value threshold.
Another aspect of the invention is the usage of an antenna diversity, i.e. two antennas in each cell or sector each providing a delay profile. Here the two delay profiles from each antenna are added and only such peaks are selected in this added profile which lie above the multiplied threshold value. Then the two delay profiles are compared separately with the multiplied threshold detected for the combined delay profile and only such paths are selected for a single antenna signal that also lie above the threshold within the respective single delay profile. The correlated evaluation of the delay profile for the path selection based on two delay profiles simultaneously is completely different to an individual consideration of the delay profiles of each antenna.
According to another aspect of the invention each cell is divided into several sectors each served by two antennas using antenna diversity. Whilst in the prior art an information must be transferred to the base station as to which sector contains the mobile station, an aspect of the invention uses a xe2x80x9cdynamic searching of the sectorsxe2x80x9d combined with individual path selections and a highly accurate softer hand-over. A xe2x80x9clocationxe2x80x9d determination of the mobile station MS can be performed based thereon.